1. Field of the Invention
The present invention relates to a process for the selective liquid-phase hydrogenation of xcex1,xcex2-unsaturated carbonyl compounds.
2. Description of the Background
Processes for the selective hydrogenation of xcex1,xcex2-unsaturated carbonyl compounds using hydrogen in the liquid phase are described, for example, in DE-A-21 14 211 and DE-A-28 39 474. Both processes operate batchwise in the presence of a palladium catalyst and a base, the process of DE-A-28 39 474 using from 15 to 50% by weight of a tertiary amine, based on the starting material, exhibiting improved selectivity and space-time yield of the hydrogenation reaction. However, even this improvement does not give satisfactory reaction times. Although the reaction times can be shortened by using larger amounts of the palladium catalyst, this is, however, uneconomical because of high catalyst costs, and in addition larger amounts of catalyst are disadvantageous in view of the problems of handling solids. Neither is successful regeneration of the deactivated catalyst generally possible.
Accordingly, one object of the present invention is to provide a method of hydrogenating xcex1,xcex2-unsaturated carbonyl compounds of the formula (I) to the corresponding xcex1,xcex2-unsaturated carbonyl compounds of the formula (II) in a more economical manner, while improving the space-time yield and reducing the investment costs.
Another object of the present invention is to provide a method of more economically hydrogenating citral to citronellal.
Briefly, these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for the selective liquid-phase hydrogenation of xcex1,xcex2-unsaturated carbonyl compounds of the formula (I) 
where R1 is hydrogen or an organic radical, and R2, R3 and R4, independently of one another, are hydrogen or a C1- to C4-alkyl group,
to saturated carbonyl compounds of the formula (II) 
with hydrogen in the presence of a pulverulent palladium and/or rhodium catalyst and in the presence of an organic base, which comprises conducting the hydrogenation in a packed bubble column reactor in which the product is recycled and the hydrogen gas is recirculated.